i have the same problem, i hvae a 1gb pen drive, and until today it has stopped working, but i think the reason was that i had put it into a usb port that was for some reason connected to a 8V thingy on my motherboard - so i think its fried my pen drive -.-
now it wont let me access or format it, although it still finds the drive.
i dont really have an answer, but check your usb ports as one of them might be faulty.
Much Love
Sporky, February 2008